There are a plurality types of detectors that are provided for an electron microscope and use different detection methods. A scanning electron microscope having especially a low-vacuum observation function may include, in addition to a secondary electron detector used in a high vacuum of 1 Pa or less, a backscattered electron detector and a low-vacuum secondary electron detector. The backscattered electron detector serves as means for detecting a signal in a low vacuum of 1 Pa or higher and detects an backscattered electron from the surface of a specimen by irradiation of the surface of the specimen with an electron beam. The low-vacuum secondary electron detector uses an amplification effect in which such a process is repeated that a secondary electron that is generated from the surface of a specimen by irradiation of the surface of the specimen with an electron beam collides with a gas molecule remaining in a specimen chamber so that the gas molecule is divided into an electron and a positive ion.
To switch signals detected by the detectors, in addition to the switching operation, such operations are required as inserting the detectors in the specimen chamber and setting a working distance, a vacuum level, an acceleration voltage, and the diameter of an electron beam and operations for these settings are complicated. For example, from the perspective of a theoretical resolution that depends on the diameter of the electron beam, it is preferable that the working distance and the diameter of the electron beam be set to small values. When the backscattered electron detector is used, if the working distance is small, an electron that is specularly reflected near the electron beam deviates from an electron beam path of the detector to propagate upward, resulting in reducing in the detection efficiency. In addition, since reducing the diameter of the electron beam means reducing the amount of a current of the electron beam, the amount of secondary electrons generated by irradiation of the specimen with the electron beam is reduced and the amount of backscattered electrons from the specimen by the irradiation is reduced. Therefore, the optimal working distance and the optimal diameter of the electron beam are empirically determined on the basis of the relationship between the detection efficiency and the diameter of the electron beam. As for a vacuum level, the low-vacuum secondary electron detector uses amplification of gas that remains in the specimen chamber. Therefore, although the efficiency of detecting a signal is higher in a low vacuum, the electron beam used for irradiation of the specimen more largely scatters as the vacuum level decreases, resulting in a deterioration of the quality of an image when the vacuum level is lower than a certain value. As for an acceleration voltage, although the resolution is generally higher as the acceleration voltage is higher, the acceleration voltage is set, in considerations of damage to the specimen and of influence of electric charges, within a range of low value of acceleration voltage in which the generation efficiency of secondary electrons and backscattered electrons, which are generated by the irradiation of the specimen with the electron beam, is sufficiently obtained. In addition, as the acceleration voltage is higher, the location at which the secondary electrons are excited is deeper from a surface layer of the specimen. As the acceleration voltage is lower, the location at which the secondary electrons are excited is shallower from the surface layer of the specimen. Thus, since the quality of an image varies depending on the acceleration voltage, the acceleration voltage is adjusted to obtain a desired quality of the image. However, when the acceleration voltage is lower than the low value that allows the generation efficiency of secondary signals to be sufficiently high, the amount of the secondary signals is reduced, and the quality of the image is not satisfactory due to noise occurring in the image. The detectable lower limit of the intensities of the secondary signals, i.e., the lower limit of the acceleration voltage that allows the quality of an image to be satisfactory, depends on the detection sensitivity of the detector.
A technique described in Patent Document 1 is known as an example in which insertion of detectors is improved in mechanism. However, it is necessary to separately set observation conditions such as a working distance and a vacuum level. For comparison with the qualities of images expressed by signals detected by detectors, it is necessary to repeat switching of the detectors and setting of the observation conditions for each of observations.